Photothermography is an established imaging technology. In photothermography, a photosensitive media is exposed to radiation to create a latent image which can be thermally processed to develop the latent image. Devices and methods for implementing this thermal development process are generally known and include contacting the imaged photosensitive media with a heated platen, drum or belt, blowing heated air onto the media, immersing the media in a heated inert liquid and exposing the media to radiant energy of a wavelength to which the media is not photosensitive, e.g., infrared. Of these conventional techniques, the use of heated drums is particularly common.
A common photosensitive media usable in these imaging processes is known as a photothermographic media, such as film and paper. One photothermographic media has a binder, silver halide, organic salt of silver (or other reducible, light-insensitive silver source), and a reducing agent for the silver ion. In the trade, these photothermographic media are known as dry silver media, including dry silver film.
In order to precisely heat exposed photothermographic media, including film and paper, it has been found to be desirable to use electrically heated drums. In apparatus employing this technique, a cylindrical drum is heated to a temperature near the desired development temperature of the photothermographic media. The photothermographic media is held in close proximity to the heated drum as the drum is rotated about its longitudinal axis. When the temperature of the surface of the heated drum is known, the portion of the circumference around which the photothermographic media is held in close proximity is known and the rate of rotation of the drum is known, the development time and temperature of the photothermographic media can be determined. Generally, these parameters are optimized for the particular photothermographic media utilized and, possibly, for the application in which the photothermographic media is employed.
In order to achieve a high quality-image in the photothermographic media, very precise development parameters must be maintained. Generally, the circumference of the drum over which the photothermographic media travels will not vary significantly. Also, the rate of rotation of the drum, or the transport rate of the photothermographic media through the thermal processor, can be rather precisely maintained. However, it is generally more difficult to control and maintain the temperature of the surface of the drum.
In addition, other factors also contribute to inaccurate processing. The closeness of the proximity which the photothermographic media is held to the drum partially determines the temperature at which the emulsion in the photothermographic media is heated. Further, the presence of foreign particles between the drum and the photothermographic media can interrupt the flow of heat from the drum to the photothermographic media which can affect image quality.
Because many factors affect image quality, one of which is the temperature at which the photothermographic media is developed, the preciseness at which the surface temperature of the drum can be maintained is important to thermal processing of photothermographic media.
The temperature of the drum depends upon many factors. These include the rate at which heat is delivered to the drum, the thermal conductivity and the thermal mass of the drum, the thermal mass of the photothermographic media, the rate, i.e., the number of sheets (if sheet photothermographic media is used) of photothermographic media being processed, the ambient temperature, whether thermal processing is just beginning or whether the thermal processing is in the middle of a long run.
In addition, heated drums are used extensively in various other material processing applications. Examples include calendaring, laminating, coating and drying.
Typically, heat is delivered to such drums through the use of electrical resistance heating elements. Since the heated drum is rotating during thermal processing and since it is a desirable to deliver electrical power to the electrical resistance heating elements during rotation of the drum, is desirable to be able to deliver electrical power from a stationary power source, e.g., the standard AC line, to the moving, rotating drum. Electrical power may be delivered to the drum through the use of slip rings coupled to the drum.
In addition, to precisely control the temperature of the electrically heated drum there should be a means to sense the temperature of the drum and a means to control the electrical power applied to the electrical resistance heaters in response to the signal from the temperature sensor.
U.S. Pat. No. 5,580,478, issued Dec. 3, 1996, inventors Tanamachi et al., discloses such a heated drum processor where separate electrical resistance heaters heat a central heat zone and contiguous edge zones. Temperature control of the electrical heaters is obtained through duty cycle modulation. Solid state relays in the power circuit to the electrical heaters are turned on and off with zero crossing triggering.
Power transients cause flickering light in the lighting systems that share the affected power grid. New flicker suppression standards have become law in recent years in Europe. The common practice in flicker suppression has been to add suppression electronics to an apparatus thus increasing manufacturing costs. A common solution has been to use the AC input power and its zero crossing as a reference as to when to adjust throughput power to load. U.S. Pat. No. 4,908,956, issued Mar. 20, 1990, inventor Grund, U.S. Pat. No. 5,907,743, issued May 25, 1999, inventor Takahashi, U.S. Pat. No. 6,188,208, issued Feb. 13, 2001, inventors Glaser et al., are examples. They differ in how to adjust the power on/off timing relative to the zero crossing. All use electronic circuits to implement. Another invention, U.S. Pat. No. 5,818,208, issued Oct. 6, 1998, inventors Othman et el., uses electronics to measure the AC power voltages and load currents at the voltage source converter terminal and calculate active and reactive current loads in an effort to minimize flicker.
U.S. Pat. No. 6,420,685B1, issued Jul. 16, 2002, inventor Tanamachi discloses a control system for reducing flicker in an electrical resistance heater, a bidirectional solid state switching device connected between the source and the electrical resistance heater; and a control circuit for controlling the bidirectional solid state switching device to supply a varying, phase controlled duty cycle of current to the heater which effectively ramps heater power up and down in response to a binary control signal which randomly turns on the switching device.
There is thus a need for a way to control flicker in an electrical heater system that is efficient and cost effective.